For example, some fish species have been classified as macrosmatic or microsmatic, depending on the morphological characteristics of their olfactory systems (Atta 2013). This comparative approach led researchers to conclude that structural morphology can predict ecological adaptations (Kasumyan 2004 Schluessel et al. Since Burne, the field has grown rapidly to elucidate the complexity and diversity of fish olfaction, and how basic design features are evolutionarily well conserved among fish and other vertebrates (Kasumyan 2004 Hansen and Zielinski 2005). This remarkable paper was the first to highlight the astonishing diversity of fish olfactory systems. With this study, we provide a foundation for future research into the sensory ecology of this imperiled fish.Ĭomparative anatomy of fish olfactory systems was established as a field of study more than a century ago when one of the most thorough reviews was published by Burne ( 1909). Although the sense of smell has been overlooked in Delta Smelt, we conclude that the olfactory epithelium has many characteristics of macrosmatic fish. The olfactory neurons were supported by robust ciliated and secretory sustentacular cells. Ciliated, microvillous and crypt neurons were clearly identified using morphological and immunohistochemical features. Calretinin immunohistochemistry revealed a diffuse distribution of olfactory receptor neurons within the epithelium. We show that Delta Smelt have a multilamellar olfactory rosette with allometric growth. We provide a description of the gross morphology, histological, immunohistochemical, and ultrastructural features of the olfactory rosette in this fish and discuss some of the functional implications in relation to olfactory ability. Their olfactory biology, in particular, is poorly understood and a basic description of their sensory anatomy is needed to advance our understanding of the sensory ecology of species to inform conservation efforts to manage and protect them. The Delta Smelt ( Hypomesus transpacificus) is a small, semi-anadromous fish native to the San Francisco Bay-Delta Estuary and has been declared as critically endangered.